Will Mark Glenn hair extensions damage my own hair?
No. Damage can only occur if they're applied incorrectly, applied on hair unsuitable for extensions or left in too long before either being taken out or replaced.
It's our job to make sure you get the care and expertise you're paying for. And it's your job to make sure you don't leave them in for more than about 3 months before having them either removed or replaced, just like any other extension method.